The Cadava potion is a one-dose potion that cannot be made using the Herblore skill. It can only be made by taking Cadava berries to the Apothecary in Varrock. Cadava berries are known to be very poisonous, thus, a potion with Cadava berries can be very dangerous to drink. The potion can only be drunk while in the Apothecary's house and, when drunk, will cause the player to black out for a short amount of time. Drinking this potion will not result in any actual damage to the player, nor will it poison the player.
A Cadava potion is used in the Romeo & Juliet quest. The player would give the Apothecary Cadava berries in order to receive a Cadava potion. The Cadava potion was then used to fake Juliet's death so Romeo could meet her.
Drinking this potion will give the message "Oh dear. You are almost dead" similar to the message you get when dying saying "Oh dear. You are dead."